ORIGIN OF THE LATERAL LINE PRIMORDIA 595 



can be no question of the distinct origin of this Hne entirely sep- 

 arate from the preauditory placode, since its first appearance is 

 entirely distinct, not only from the preauditory placode, but also 

 from the three structures associated with the formation of the 

 gill slit. 



The period between the 106-hour stage and the 120^ hour stage 

 is marked by the appearance of definite supra-orbital and infra- 

 orbital lines, (figs. 13 and 15). The anterier end of the primor- 

 dium bifurcates and from the bifurcated ends the two sensory 

 lines originate. The mode of origin of the supra-orbital and 

 infra-orbital lines is shown in figures 9, 10, 11 and 12, which are 

 drawn from an embryo of 137 hours, in which the development 

 was below normal, so that it may be taken to represent condi- 

 tions prior to the previous stage (120| hours). It will be seen 

 that the supra-orbital and infra-orbital lines appear to be cephalad 

 continuations of the common supra-branchial primordium of the 

 lateral lines. 



After the appearance of the supra-orbital and infra-orbital 

 lines, the primordium degenerates posterior to the point of origin 

 of the lines, and in later stages, we could find no evidence of an 

 anastomosis between the supra-orbital and sub-orbital lines. 



In the 120^-hour stage (fig. 5), the supra-orbital line is 310 ^ 

 in length. It begins at a point 90 ix in front of the anterior limit 

 of the auditory vesicle and directly dorsal to the median point 

 in the line of contact between ectoderm and endoderm in the 

 region of the hyoid gill pocket. From this point it may be 

 traced cephalad and dorsad to a point 50 ix behind the posterior 

 boundary of the optic vesicle and at the level of the dorsal 

 limit of the flexure of the brain-floor immediately below the 

 mesencephalon. 



The infra-orbital line begins at a point 30 /z in front of the pos- 

 terior limit of the supra-orbital line at a slightly lower level and 

 distinct from it. From this point it may be traced forward for 

 100 IX, then cephalad and ventrad until it ends 30 ix behind the 

 posterior boundary of the optic vesicle and at the level of the 

 hypophysis. The extent of the lines and their relation to sur- 

 rounding structures is shown in figure 5. 



